1. Field of the Invention
The present invention relates to a method and apparatus for conversion of paper residue into a mineral product.
2. Description of the Related Art
Such a method and apparatus are known from practice and are taught by International Publication W096/06057. The term “paper residue” as used herein includes both paper sludge resulting from the industrial production of paper, and paper waste such as paper having on average fibers which are too short. The paper residue may also include deinking sludge. The mineral product obtained with this method and installation can be used, for instance as replacement for cement or as a sorbent for removal of metals from (hot) gas streams.
The known method employs a fluidized bed device that forms part of the installation, whereby below the fluidized bed device there is a distribution plate for securing an even distribution and supply of at least combustion air to the bed material and to the paper residue that is introduced into the fluidized bed device for conversion to the mineral product.
An air box is provided below the distribution plate for supplying the combustion air to the bed material and to the paper residue above the distribution plate. The air box may in certain situations, in addition to the combustion air, also supply cleansed recirculated flue gases.
Although it is not necessary, it may be preferable from a viewpoint of energy-efficiency that a heat exchanging section is employed which receives in a first part thereof (ambient) air—which may be supplemented with recirculated flue gases—, and in a second part separate from the first part flue gases from the fluidized bed device for exchanging heat between the flue gases and the (ambient) air for heating the latter. The heat exchanging section is then further connected to the air box for supplying the heated (ambient) air to the air box for use as combustion air.
When employing the method and installation for converting paper residue into the mineral product continuously, the problem occurs that the quantity of bed material and the dimensions of its particles vary, causing the duration of uninterrupted or undisturbed operation of the installation to be limited. It is a known problem in the prior art that the amount of bed material and the diameter of its particles increase to a level that measures have to be taken. At times it is possible to remove during operation of the installation the deteriorated bed material and replace it with bed material having the appropriate properties. At other times this is not possible, in which case it is required to interrupt operation of the installation in order to allow the bed material to be replaced for material having the required specifications.
The proper operation of the installation for conversion of paper residue requires that the bed material and the diameter of its particles are maintained at a specified level. This specified level may vary a little between installations. The optimum level that applies to a specific installation may therefore need to be determined on a moderate trial and error basis. Generally speaking, the parameters that are desirable for the bed material and the diameter of its particles that are capable to entertain desirable fluidization conditions in the fluidized bed at fluidization velocities of more than 0.5 meter per second, are that the particles are maintained at a diameter between 0.7 and 4 millimeters (mm), for example between 1.2 and 1.8 mm. The height of the fluidized bed should be maintained at a level so that the pressure difference between a location immediately above the distribution plate and the freeboard area will be in the range between 40-200 centimeters (cm) water column. The bed material is further maintained at a level of its spherical shape factor of approximately 0.8.
The just-mentioned spherical shape factor, or sphericity, has been introduced in the general literature on fluidized bed combustion to account for deviation from the ideal spherically shaped particle. It is customary to define the sphericity as the ratio of the surface area of a perfect sphere and the surface of the particle under consideration, whilst both particles have identical volumes:Sphericity=As/Ap and 0<sphericity<1,wherein As indicates the surface area of a perfect sphere, and Ap indicates the surface area of the particle that is considered, and wherein both particles have identical volumes.
Application of basic mathematics regarding the surface area and the volume of an ideal sphere results in:Sphericity=((4*pi*(3/(4*pi))^(⅔))*Vp^(⅔))/Ap, wherein Vp is the volume of the particle under consideration and Ap is the surface area of this particle.
It goes without saying that replacement of the bed material goes at the expense of production quantity, whereas maintaining the conversion process with the deteriorated bed material goes at the expense of production quality. Both have a financial impact.
The prior art, notably GB-A-1 474 711 and JP 58069314, disclose fluidized bed devices in which it is known to control the feed rate of combustion air.
What is needed in the art is a method and apparatus for maintaining the bed material as used in a fluidized bed within specification, so as to avoid the necessity of interrupting or disturbing the continuous process of converting paper residue into a mineral product and to keep both production rate and production quality as high as possible.